How to check if a cmdlet exists in PowerShell at runtime via script

If the command is in Verb-Noun form then you can use Get-Command with Verb and Noun parameters.

# Usage:
if (Get-Command -Verb Invoke -Noun MyCommand) {
  # cmdlet Invoke-MyCommand exists
}

---

Get-Command -Verb Get -Noun Item

# Output:
# CommandType     Name                  Version    Source
# -----------     ----                  -------    ------
# Cmdlet          Get-Item              7.0.0.0    #Microsoft.PowerShell.Management

Get-Command -Verb Take -Noun One

# No output.

function Take-One { [CmdletBinding()]param() }
Get-Command -Verb Take -Noun One

# Output:
# CommandType     Name                   Version    Source
# -----------     ----                   -------    ------
# Function        Take-One

Tested on Windows PowerShell 5.1 and PowerShell Core 7.0.

Edit 2020-11-09 Additional example. Also usage example (adapted from Keith Hill answer).

This is a simple function to do what you're like to do :)

function Check-Command($cmdname)
{
    return [bool](Get-Command -Name $cmdname -ErrorAction SilentlyContinue)
}

How to use (for example):

if (Check-Command -cmdname 'Invoke-WebRequest')
{
     Invoke-WebRequest $link -OutFile $destination
}
else
{
     $webclient.DownloadFile($link, $destination)
}

Use the Get-Command cmdlet to test for the existence of a cmdlet:

if (Get-Command $cmdName -errorAction SilentlyContinue)
{
    "$cmdName exists"
}

And if you want to ensure it is a cmdlet (and not an exe or function or script) use the -CommandType parameter e.g -CommandType Cmdlet